glusterd: More checks before starting rebalance/remove-brick

Check if a previous remove-brick operation has been committed before
starting a new rebalance/remove-brick task.

Change-Id: I553e5ba64a6a352ca91032ab1a17997051a4494e
BUG: 963541
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5019
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>